The Japanese team for the Fed Cup matches are:
Naomi Osaka (singles ranking 22, doubles ranking 902)
Kurumi Nara (s 100, d 444)
Miyu Kato (s 137, d 55)
and Makoto Ninomiya (s 612, d 40)
I assume that the first two will be the singles players and the last two the doubles players.
Looking at the rankings for the British team, Johanna Konta will be a singles player, Heather Watson could be both singles and doubles, Gabriella Taylor if she plays more likely to be singles than doubles, and Anna Smith a cert for doubles.